During the last year studies have continued on changes on the active site of cardiac myosin with physical training. Myosin was treated with trypsin and the heavy meromyosin was found to have a higher ATPase activity in preparations from conditioned hearts than in preparations from sedentary hearts. In the presence of 8-anilino-1- napthalene sufonic acid, heavy meromyosin from conditioned hearts had a greater fluorescence than from sedentary hearts and different responses to ATP, EDTA, and urea, all suggesting an alteration at the active site. We are further exploring the active site quantitatively with C14- iodoacetamide incorporation. Sarcoplasmic reticulum preparations were standardized from rat hearts, and we have initiated studies on the uptake and binding of calcium from hearts of sedentary and conditioned rats. Studies on deconditioning of the heart in relationship to actomyosin ATPase activity have also been begun. This has been a tooling up year so that the number of statistically valid results are few, but the laboratory is now in full production on this project.